This invention is directed generally to microfluidic devices that employ electroosmotic flow, and particularly electrokinetic pumps, having flow channels that contain a stationary phase or porous dielectric material prepared by an inverse emulsion method that imparts desirable structure and properties.
Monolithic polymeric materials composed of polymerized monomers (styrenes, acrylates, methacrylates, etc.) have proven useful as the stationary phase for various chromatographic applications and particularly for applications involving miniaturized or capillary columns where traditional methods of column packing have proven to be ineffective. Polymer materials are among the class of materials that have been found to be useful for electric field-driven applications and particularly as porous dielectric media for electrokinetic pump applications. Porous stationary phase materials that are “cast-in-place” or “cast-to-shape” by phase-separation polymerization of mixtures of monomers directly within the confines of a chromatographic column, such as those disclosed in U.S. Pat. No. 5,728,457 entitled “Porous Polymer Material with Gradients” and issued to Frechet et al. on Mar. 7, 1998, have been developed to address this problem. By careful control of polymerization rate, time, and temperature Frechet has produced a single molded polymer monolith that possesses desirable hydrodynamic properties by virtue of being traversed by large channels and permeated by small pores. Several variations have already been successfully used in the separation of polyaromatic hydrocarbons (PAH), PTH-labeled amino acids, peptides, and explosives.
In phase-separation polymerization, a solution of monomers is polymerized. When the polymer molecules grow sufficiently large, they separate from the inert solvent (phase separate). A liquid-liquid or liquid-solid phase separation can occur with partitioning of the unreacted monomers. If a three-dimensional network is formed before precipitation, a polymer monolith consisting of a three-dimensional network of solid polymer and an interconnected network of solvent filled pores will be formed. The structure and dimensions of the interconnected porous polymer network can generally be determined by controlling the proportions of solvent as well as the monomer and solvent composition. However, prior art phase separation processes for producing polymer stationary phase material are very difficult to control completely since the polymer microstructure is determined principally, and irreversibly, by the conditions that prevail at the time of phase separation. Thus, if the desired network structure has not formed, the polymer can precipitate as a particulate material. Further, an undesired structure cast by prior art phase-separation methods into an intricate substrate either cannot be removed or can only be removed with great difficulty, generally requiring the substrate to be completely refabricated.